U.S. patent number 4,540,887 [Application Number 06/461,907] was granted by the patent office on 1985-09-10 for high contrast ratio paper sensor.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Robert E. Crumrine, Timothy M. Minerd.
United States Patent |
4,540,887 |
Minerd , et al. |
September 10, 1985 |
High contrast ratio paper sensor
Abstract
A high contrast ratio sensor is provided by introducing a pair
of photodetectors into the sensor circuitry. One of the detectors
is a diffuse detector and the other is either a reflective or
transmittance detector. The two photodetectors are connected in a
manner to cause the diffuse detector output to subtract from the
reflective (or transmissive) detector output. This subtraction of
signals provides a high contrast ratio. In a first embodiment, a
diffuse detector and a reflectance detector are located on the same
side of the paper as the emitter light and in a second embodiment a
diffuse detector and a transmittance detector are located on
opposite sides of the paper to be sensed.
Inventors: |
Minerd; Timothy M. (Pittsford,
NY), Crumrine; Robert E. (East Rochester, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23834424 |
Appl.
No.: |
06/461,907 |
Filed: |
January 28, 1983 |
Current U.S.
Class: |
250/559.4;
250/223R; 250/559.11; 250/559.16 |
Current CPC
Class: |
G01N
21/4738 (20130101); B41J 11/009 (20130101) |
Current International
Class: |
B41J
11/00 (20060101); G01N 21/47 (20060101); G01N
021/86 () |
Field of
Search: |
;250/221,222.1,223R,571,572,559,562,563,561 ;356/446,448,443-445
;271/258,259,262,263 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nelms; David C.
Attorney, Agent or Firm: Chapuran; Ronald F.
Claims
What is claimed is:
1. Apparatus for determining the presence of paper in a paper path
including:
a source of light disposed near the paper path for emitting light
in the direction of the paper path,
a reflecting surface,
a first detector disposed near the paper path for detecting a first
quantity of light reflected from the reflecting surface when no
paper is in the paper path and for detecting a lesser quantity of
light reflected from the reflecting surface when paper is in the
paper path, the paper in the paper path diffusing a portion of the
light emitted by the light source, the improvement comprising:
a second detector disposed near the paper path to detect light
emitted from the source of light and diffused by paper in the paper
path, the second detector being electrically connected to the first
detector.
2. The apparatus of claim 1 wherein contrast ratio is defined as
the ratio of the output current of the first detector without paper
in the paper path to the output current with paper in the paper
path and the first detector is a phototransistor, the second
detector being electrically connected to the base of the
phototransistor whereby there is provided an increase in the
contrast ratio of the apparatus with the use of the second
detector.
3. The apparatus of claim 2 wherein the second detector is a
photodiode, the photodiode diminishing the output current of the
phototransistor with paper in the paper path whereby the
phototransistor senses the presence of paper in the paper path.
4. Apparatus for determining the presence or absence of paper in a
paper path including:
a light source disposed near the paper path for emitting a given
quantity of light,
a light detector disposed opposite the light source for detecting a
first quantity of light emitted from the light source with no paper
in the paper path and a second quantity of light for detecting when
paper is present in the paper path, the paper in the paper path
being disposed intermediate the light source and the light
detector, a portion of the light emitted from the light source
being diffused by the paper in the paper path, and
a diffuse detector disposed adjacent the paper path for sensing the
light that is diffused by the paper in the paper path, the light
detector being electrically connected to the diffuse detector.
5. The sensor of claim 4 wherein contrast ratio is defined as the
ratio of the output current of the light detector without paper in
the paper path to the output current with paper in the paper path
and the light detector is a phototransistor, the diffuse detector
being electrically connected to the base of the phototransistor
whereby there is provided an increase in the contrast ratio of the
apparatus with the use of the diffuse detector.
6. The sensor of claim 5 wherein the second detector is a
photodiode, the photodiode diminishing the output current of the
phototransistor with paper in the paper path whereby the
phototransistor senses the presence of paper in the paper path.
Description
This invention relates to paper sensors and in particular to a high
contrast ratio paper sensor for use in a reproduction machine.
Typical prior art sensors are shown in U.S. Pat. Nos. 3,435,240 and
4,092,068. In U.S. Pat. No. 3,435,240, the surface characteristics
of a material are determined by the quotient of output signals from
two photomultipliers. The magnitude of these signals from the
photomultipliers is determined by the simultaneous transmittance of
energy from a single light source through small and large areas of
a material being examined. In U.S. Pat. No. 4,092,068, a single
light source is directed onto a material surface and the surface
characteristics are examined by comparing the amount of light
reflected at two different angles from the surface onto a pair of
detectors.
It is also known in the prior art to provide sensors to be able to
detect translucent papers. One technique comprises a light emitter
on one side of the paper path and a transmittance detector on the
other side of the path. As paper enters the path between the
emitter and detector, the light may be attenuated sufficiently
through the paper in order that the signal sensed by the detector
indicates paper in the path. The difficulty is that the sensor
circuitry is generally tuned only to detect paper having a
particular transmittance characteristic. For papers or documents
having different transmittance characteristics, the sensor
circuitry is often insensitive to be able to detect this type of
paper without adjustments having to be made to the sensor
circuitry.
Another technique of paper sensing is to provide a reflectance
detector on the same side of the paper as the light emitter. With
paper in the path, a predetermined amount of light will be
reflected from the paper to the reflectance detector to indicate
the presence of paper. Here again, the sensing circuitry is often
sensitive only to a document or paper having certain transmittance
characteristics and for papers with different characteristics, it
is necessary to adjust the detector circuitry. This is due to the
fact that the present sensors systems have a wide range of
characteristics and therefore very low effective contrast ratios.
Because of the low contrast ratios obtained in a manufacturing or
field environment, electrical and/or mechanical adjustments are
required for different documents. The need for readjustment for
different types of papers often renders the sensors impractical for
use in applications having a wide variety of papers.
It would be desirable, therefore, to provide a relatively simple
paper sensor detector that provides a relatively high contrast
ratio and that can be used in a variety of paper sensing
applications without the need for adjustment.
Accordingly, it is an object of the present invention to provide a
relatively high contrast ratio paper sensor to be used for
detecting papers having various transmittance characteristics and
that can be used in a variety of paper sensor applications. Further
advantages of the present invention will become apparent as the
following description proceeds and the features characterizing the
invention will be pointed out in the claims annexed to and forming
a part of this specification.
Briefly, a high contrast ratio sensor is provided by introducing a
pair of photodetectors into the sensor circuitry. One of the
detectors is a diffuse detector and the other is either a
reflective or transmittance detector. The two photodetectors are
connected in a manner to cause the diffuse detector output to
subtract from the reflective (or transmissive) detector output.
This subtraction of signals provides a high contrast ratio. In a
first embodiment, a diffuse detector and a reflectance detector are
located on the same side of the paper as the emitter light and in a
second embodiment a diffuse detector and a transmittance detector
are located on opposite sides of the paper to be sensed.
For a better understanding of the present invention, reference may
be had to the accompanying drawings wherein the same reference
numerals have been applied to like parts and wherein:
FIG. 1 is a prior art reflectance detector.
FIG. 2 is a prior art transmittance detector.
FIG. 3 is a reflectance detector sensor incorporating the present
invention;
FIG. 4 is a transmittance detector sensor incorporating the present
invention;
FIG. 5 is an electrical schematic representing the configuration of
FIGS. 3 and 4; and
FIG. 6 is an alternative electrical schematic.
With reference to FIG. 1, there is shown a prior art reflectance
detector sensor. Without paper in the paper path, a sufficient
amount of light emitted from the light source 12 reflects from the
mirror 14 to the detector 16 to indicate no paper. With paper 18 in
the paper path, it is assumed that the light will be attenuated
enough by the paper so that the amount of light reflected from
mirror 14 to the reflectance detector 16 has diminished
sufficiently to indicate the presence of paper in the path.
Unfortunately, with highly translucent paper, not enough light will
be attenuated by the paper such that the detector will not
recognize the presence of the paper. Some of the light is diffused
but some is passed through the paper reflected off the mirror and
goes back up through the paper again.
FIG. 2 shows another prior art sensor embodiment. In this case, the
reflectance detector 16 has been replaced by transmittance detector
20.
In accordance with the present invention as seen in FIG. 3, an
additional detector, a diffuse detector 22 is inserted into the
sensor of FIG. 1. This detector senses light that is diffused by
the paper, that is, is not transmitted through the paper. Also,
some of the light that is transmitted through the paper and
reflected from the mirror 14 is diffused and scattered. Some of
this light also is sensed by the diffuse detector 22. It should be
noted that the diffuse detector 22 is positioned to minimize
detection by detector 22 of light reflected from mirror 14 with no
paper present. Thus, if there is a narrow light beam from source 12
so that no light is reflected from mirror 14 to detector 22, then
the position of detector 22, as shown, is suitable. This
configuration will give a high contrast ratio and a great deal of
latitude in selecting components.
If the light from source 12 is not narrow, it may be necessary to
move the detector 22 out of the range of the reflected beam. If the
detector is not moved, there will be a reduced output signal from
sensor. Therefore, there would be a decreased signal to noise ratio
and a decreased maximum achievable contrast ratio.
FIG. 4 is the prior art sensor of FIG. 2 modified in according to
the present invention. In particular, a diffuse detector 22 has
been added to sense diffused light.
FIG. 5 is an electrical schematic representing the detector of FIG.
3. In particular, the light from the suitable light source 12 (any
suitable light emitter such as an LED) is received by a transistor
24 representing the reflectance detector 16 and a diode 26
representing the diffuse detector 22. The result is a negative
feedback circuit in which one detector is looking for the absence
of light while the other is looking for the presence of light. The
signal from the diode 26 subtracts from the current through
transistor 24 and then from the output of transistor 24.
In operation, the more reflection of light from the paper, the more
light diffusion there will be. The more light diffusion means more
light will be subtracted from the amount of light received by the
reflectance detector 16. This increases the attenuation of the
light sensed by the reflectance detector 16. Also, the more light
that is detected by the diffuse detector 22, the more current is
subtracted away from transistor 24. The more the current that is
subtracted, the less the gain from the transistor 24. As shown in
FIG. 6, the diode 26 could be replaced by transistor 28.
In the ideal situation, the output signal from the transistor 24 is
0 when there is paper in the paper path and is at some high level
when there is no paper in the paper path. If the diffuse detector
22 is relatively sensitive, it will shut the transistor 24 off and
the current will be 0 through the transistor 24.
When a document enters the sensing area, the light reaching the
reflective or transmissive detectors 16, 20 is attenuated by the
document. This reduces the output current of the transistor 24.
However, the diffuse detector 22 now receives light energy and its
output current therefore increases. Since the output current from
transistor 24 is the difference between its base current due less
the output current of diode 26 or transistor 28 the total output
current from transistor 24 is greatly reduced. This increases
significantly the sensor's contrast ratio.
By proper selection of transistor and diode components, it is
possible to achieve a high contrast ratio using the present
invention. This can be understood with reference to the following
calculations: ##EQU1##
I.sub.C24 is the collector current of 24 in the configuration of
FIG. 1 or 3. ##EQU2##
Where I.sub.B1 is the base current of transistor 24 without 26 or
28 connected as in the original sensor arrangement of FIG. 1.
##EQU3##
Thus, yielding approximately a 3:1 improvement in C.R. ##EQU4##
(5) Therefore, if 1<CRX.ltoreq.CRA.CRO the maximum combined
contrast ratio is achieved.
In practice, a low performance relfective sensor could have a C.R.
as low as 20:1 (CRO) and a low performance diffusive sensor could
have a C.R. as low as 10:1 (CRA). Thus, CRX should be chosen such
that 1<CRX.ltoreq.200. Since it is desirable to have as large an
output as possible, CRX should be chosen such that
CRX>>>1.
(6) A practical range of values for CRX might be:
20.ltoreq.CRX.ltoreq.200 for a combined sensor with a maximum
achievable contrast ratio.
With reference to the calculations, ideally it is desirable to have
infinite contrast ratio. In other words, a 0 current when there is
paper in the paper path and a very high level of current when there
is no paper. The contrast ratio is simply the ratio of the current
with no paper over the current with paper. To have a high contrast
ratio, it is desirable to have very low current when there is paper
in the path.
In the contrast ratio equation (2), the numerator of the equation
(1) I.sub.24 (No Paper) has been expressed as the difference of the
base current of the transistor 24 as shown in FIGS. 5 and 6 less
the current of the diode 26 or transistor 28 or at the base of
transistor 24. The numerator of the equation is the same expression
for the current with paper in the paper path.
It is desirable to have as a rule of thumb a contrast ratio of 40.
However, assume that the contrast ratio is 4 as shown in equations
(3). That is the ratio of I.sub.B1 with no paper over I.sub.B1 with
paper is 4 to 1 and similarly the ratio of I.sub.26 paper over
I.sub.26 no paper is 4 to 1.
However, we desire a contrast ratio equal to 40 over 1.
Substituting in equation (2) the 4 to 1 contrast ratio we get
equation (4). Solving for the ratio I.sub.B1 over I.sub.26 with no
paper results in a 17.7 to 1 ratio. This determines that the ratio
of the base current in transistor 24 to the ratio of the current of
diode 26 or transistor 28 with no paper must be 17.7 to 1 to
provide a 40 contrast ratio. In other words, the light that the
detector 16 or 20 sees must be 18 times greater than the light
detector 26 sees to provide a 40/1 contrast ratio.
While there has been illustrated and described what is at present
considered to be a preferred embodiment of the present invention,
it will be appreciated that numerous changes and modifications are
likely to occur to those skilled in the art, and it is intended in
the appended claims to cover all those changes and modifications
which fall within the true spirit and scope of the present
invention.
* * * * *